1. Field of the Invention
The present invention relates to an amplifying apparatus, specifically, to an amplifying apparatus amplifying a signal including an amplitude component and a phase component.
2. Description of the Related Art
In recent years, a digital modulating method such as QPSK (Quadrature Phase Shift Keying) and multiple-value QAM (Quadrature Amplitude Modulation) has been used for wireless communication. In the QPSK and the multiple-value QAM, an amplitude (envelope) of a high frequency input signal changes as time elapses. The ratio of a peak electric power to the average electric power of a signal whose amplitude changes is referred to as PAPR (Peak-to-Average Power Ratio).
When a signal whose PAPR is large is amplified, it is necessary to cause an amplifier to operate with back off in an area in which the electric power is lower than saturated electric power to secure enough linearity for peak electric power. The efficiency of a general linear amplifier is maximum around the saturated electric power, and if the general linear amplifier operates in an area in which the back off is large, the average efficiency becomes lower.
In a wireless LAN (Local Area Network), the Orthogonal Frequency Division Multiplexing (OFDM) method is adopted to realize a multiple carrier transfer. The PAPR of this modulating method becomes larger than that of the QPSK and the multiple-value QAM, so that the average efficiency of an amplifier becomes lower.
Thus, there is need for an amplifier which operates highly efficiently even in an area in which the back off is large.
To meet the need, Envelope Elimination and Restoration (EER) is proposed as a configuration in which a signal can be amplified highly efficiently in a wide dynamic range in the area in which the back off is large (refer to “PROCEEDINGS OF THE I. R. E.” (L. Kahn), 1952, Vol. 40, pp. 803-806, FIG. 2).
FIG. 1 is a block diagram illustrating a configuration of an amplifier of the EER method. Referring to FIG. 1, an EER amplifier includes RF amplifier 901, pulse modulator 902, switching amplifier 903, low-pass filter 904, envelope detector 905, and limiter 906.
A digital-modulated high frequency analog signal which is inputted to the EER amplifier is caused to branch to two signals. One of the two signals is inputted to envelope detector 905, and the other signal is inputted to limiter 906.
Envelope detector 905 eliminates a carrier frequency component from the inputted signal to extract an amplitude component (envelope). An output of envelope detector 905 is inputted to pulse modulator 902. Pulse modulator 902 pulse-modulates the inputted signal to output the signal to switching amplifier 903. Switching amplifier 906 amplifies current of the signal from pulse modulator 902 by turning on/off VCC to output the signal to low-pass filter 904. Low-pass filter 904 filters the signal from switching amplifier 906. The output from low-pass filter 904 becomes an analog amplitude signal which is obtained by amplifying the amplitude signal outputted from envelope detector 905, and is delivered as an electric power source to RF amplifier 901.
On the other hand, limiter 906 to which the other branched signal is inputted converts the input signal to a phase signal, whose amplitude is constant, and which includes only phase information, and inputs the signal to RF amplifier 901.
RF amplifier 901 amplifies the phase signal from limiter 906 by using the amplified amplitude signal from low-pass filter 904 as an electric power source. Thereby, the amplitude signal from low-pass filter 904 and the phase signal from limiter 906 are mixed, and the mixed signal becomes a high frequency output signal which is obtained by amplifying the input signal to the EER amplifier.
According to the EER amplifier, RF amplifier 901 can be caused to constantly operate in the vicinity of the saturated electric power in which the efficiency is the maximum. Referring to a configuration of the amplitude signal side, it is sufficient that pulse modulator 902 processes a signal in a logic level, so that the electric power consumption of pulse modulator 902 is small. Switching amplifier 903 only turns on/off electric power source VCC as a switching operation, thus, amplifier 903 operates ideally with the efficiency of 100%. Low-pass filter 904 can be configured with lossless inductors and capacitances.
Therefore, as compared with a case in which RF amplifier 901 independently operates in a class-A operation or a class-B operation, the EER amplifier can highly efficiently amplify the digital-modulated high frequency input signal across a wide dynamic range.
In addition, Envelope Tracking (ET) is also known as another configuration which can highly efficiently amplify signals even in an area in which the back off is large (refer to e.g. “IEEE MTT-S Digest” 2000, Vol. 2, pp. 873-876, FIG. 1). A configuration of the ET amplifier is a configuration in which limiter 906 is eliminated from the EER amplifier illustrated in FIG. 1.
RF amplifier 901 does not operate as saturated, but operates linearly, so that the efficiency of the ET amplifier is slightly lower than that of the EER amplifier. However, as in the EER amplifier, the ET amplifier also changes an electric power source voltage of RF amplifier 901 according to the output electric power of the amplitude signal side including a pulse modulation and switching amplification, and delivers only required minimum DC electric power even in the back off area to RF amplifier 901. Thus, the ET amplifier can highly efficiently amplify signals as compared with a case in which RF amplifier 901 independently and linearly amplifies signals with a fixed electric power source.
While pulse width modulation (PWM), as a modulating method of a pulse modulator which are used in the EER amplifier and the ET amplifier, has been used as a general pulse modulating method, as another case, a configuration is proposed in which a delta modulation (or, pulse density modulation (PDM)) is applied, whose linearity is excellent (refer to Japanese Patent No. 3207153 (pp. 8, FIG. 3) U.S. Pat. No. 5,973,556 (pp. 3, FIG. 3).
FIG. 2 is a block diagram illustrating a configuration of another EER amplifier in which the delta modulation is applied. Referring to FIG. 2, another EER amplifier includes amplitude path 911 and phase path 920.
Amplitude path 911 includes delta modulation amplifier 910 and envelope detector 912. Delta modulation amplifier 910 includes envelope detector 913, difference detector 914, quantizer 915, class-D amplifier 916, low-pass filter 917, and attenuator 918.
Phase path 920 includes limiter 921, non-linear front amplifier 922, and output stage amplifier 923.
Delta modulation amplifier 910 of amplitude path 911 attenuates a high frequency output from output stage amplifier 923 of phase path 920 with attenuator 918, and extracts an amplitude component with envelope detector 913. Difference detector 914 obtains the difference between an amplitude component of a high frequency input detected by envelope detector 912 and an amplitude component of a high frequency output detected by envelope detector 913. Quantizer 915 quantizes the difference, and class-D amplifier 916 amplifies the quantized signal. An output of class-D amplifier 916 is filtered by low-pass filter 917, and is delivered as an electric power source to output stage amplifier 923 of phase path 920.
In phase path 920, limiter 921 extracts a phase component from the high frequency input, and non-linear front amplifier 922 amplifies a signal of the phase component. Output stage amplifier 923 finally amplifies an output of non-linear front amplifier 922, and generates a high frequency output.
As described above, the linearity of the EER amplifier can be improved by using a delta modulating method whose linearity is excellent.
However, there is a problem that a noise level of an amplifying apparatus using the EER method or the ET method is higher than that of a normal linear amplifying apparatus.
For example, this is because in pulse modulator 902 of the EER amplifier illustrated in FIG. 1, noise is induced when an analog signal is converted to a pulse signal. When pulse width modulation is used as a pulse modulating method, a switching noise is induced which corresponds to a cycle of a reference triangle waveform signal. When the delta modulation is used as the pulse modulating method, a white quantization noise is the main noise source.
The noise induced in pulse modulator 902 is reduced to some extent by low-pass filter 904. However, the noise is not completely eliminated, and the amplitude signal which is superimposed with remaining noise is mixed with a phase signal in RF amplifier 901.
As a result, a noise component is mixed in an output signal, and SNR (Signal to noise ratio) of a spectrum of the output signal is degraded. In wireless communications in recent years, such as mobile telephones, using digital modulation, it is specified in a communication standard that an adjacent channel leakage power ratio (ACPR) will be suppressed to a low level. If the SNR is degraded by noise induced in pulse modulator 902, the specification for the ACPR may not be satisfied.
To improve the SNR of pulse modulator 902, in the pulse width modulating method, it is effective to increase the frequency (switching frequency) of the reference triangle waveform signal which is used for comparison with an input signal. In the delta modulating method, it is effective to increase a sampling frequency, and increase an over-sampling ratio. The over-sampling ratio is a ratio of a sampling frequency to a frequency which is twice the input signal band.
However, if these techniques are adopted, problems will occur in which electric power consumption of a signal processing circuit in pulse modulator 902 is increased, loss induced when switching amplifier 903 is switched is increased, and electric power consumption of the entire EER amplifier is increased.
As described above, there exists a trade off between the increase of the SNR and the decrease of electric power consumption in the pulse modulating method.
On the other hand, it is assumed that the EER amplifier illustrated in FIG. 2 can correct an error attributed to the non-linearity of delta modulation amplifier 910 and output stage amplifier 923 by providing feedback from output stage amplifier 923 to delta modulator 910. However, a negative feedback loop is configured to be returned to delta modulator 910, so that quantization error which is induced by quantizer 915 in delta modulator 910 is superimposed again. Thus, it is thought that waveform distortion arise from non-linearity can be reduced but noise can not be eliminated in principle with this configuration.